1. Field of the Invention
The present invention generally relates to radio frequency identification tags and antennas for the radio frequency identification tags.
2. Description of the Related Art
Recently, a system using a radio frequency identification tag has become widely noticed. This system includes a radio frequency identification tag and a reader/writer device. The reader/writer device can read out information from the radio frequency identification tag and write information into the radio frequency identification tag. Such a reader/writer device may be called an interrogator. The radio frequency identification tag may be called an RF tag, an RFID, an RFID tag, an IC tag, an electronic tag, or the like.
There are an active type and a passive type in the radio frequency identification tags. The active type RF tag can provide electric power by itself so that a device structure at a side of an RF tag reader can be simplified.
The passive type RF tag cannot provide electric power by itself and therefore an operation such as sending ID information is performed by receiving energy from the outside. The passive type RF tag is preferable from the view point of cost saving of the radio frequency identification tag and will be useful.
From the view point of the frequency band of a signal to be used, there are an electromagnetic coupling and an electromagnetic wave type in the radio frequency identification tags.
The electromagnetic coupling type radio frequency identification tag uses a frequency band of several kHz, a frequency band of approximately 13.5 MHz, or the like. The electromagnetic wave type radio frequency identification tag uses a Ultra-High Frequency (UHF) band such as approximately 950 MHz or a high frequency band such as approximately 2.45 GHz.
From the view point of an increase in communicating distance, it is preferable to use the high frequency.
In the system using the radio frequency identification tag, data such as identification information (ID) or a product number are read out and written in via the radio frequency identification tag, so that product management or the like may be performed. Alternatively, information indicating a value such as a ticket or points may be written in or read out. Thus, the radio frequency identification tag is expected to be widely used for not only mere product management but also as an electronic ticket for a next generation traffic system, electronic money, and others.
There may be several subjects accompanying the radio frequency identification tag. Particularly, whether the subject has conductivity is critical for design of the radio frequency identification tag. If the subject has insulating properties, operating characteristics or the communication environment of the radio frequency identification tag does not have to be changed before and after the radio frequency identification tag is attached to the subject.
However, if the radio frequency identification tag is attached to a conductive material such as a metal housing, an imaginary (complex) electric current due to the conductivity is generated at the time of communication. Therefore, a large influence is given to a receiving signal level and/or transmitting signal level so that the operating characteristics of the radio frequency identification tag are drastically changed.
For example, a metal container or a stretcher formed by a metal pile may correspond to a specific example of a conductive subject. But the conductive subject is not limited to these examples.
Because of this, the communicating distance of the radio frequency identification tag attached to the conductive subject is limited to be shorter than that of the radio frequency identification tag attached to the subject having the insulating properties.
In order to solve the problem discussed above, a passive type radio frequency identification tag wherein a configuration of the antenna is devised so that the tag can be directly attached to metal is suggested in “Development of a tag for metal attachment and a reader/writer of RFID in conformity of EOC ClG2 standard”, Mitsubishi Electric Corporation, Sep. 7, 2005, posted at http://www.mitsubishielectric.co.jp/news-data/2005/pdf/0907-b.pdf.
However, an antenna for a radio frequency identification tag disclosed in the above-mentioned publication is relatively thick such as approximately 5 mm. This is not preferable from the view point of manufacturing the radio frequency identification tag at low cost. In addition, in the radio frequency identification tag disclosed in the above-mentioned publication, a thin and long groove (slit) is formed in a metal plate forming the antenna so as to pierce the metal plate and a radio frequency identification tag integrated circuit is to be connected into the slit. It is not easy to speedily manufacture such a complex configuration of the antenna with high precision.